A large proportion of ceramic ware is produced by casting or mechanical working in permeable or porous molds or dies. In general, such molds have been made of plaster of paris because it possesses the excellent permeability and porosity necessary for producing ceramic ware. The pores of the plaster of paris absorb the water from the clay or similar material utilized for ceramic ware forming. More particularly, plaster of paris molds are utilized almost exclusively in the so-called Ram process wherein moist clay is pressed between mold halfs and the moisture enters the pores of the plaster of paris mold. The resulting pressed clay is then released from the mold by passing a fluid such as gas (e.g. air) through the pores in the mold.
Even though plaster of paris and other gypsum based materials have been relatively satisfactory for use in producing porous or permeable molds they nevertheless have significant drawbacks which, in general, are low mechanical strength, low abrasion resistance, low thermal shock resistance, and significant chemical solubility in water. These deficiencies in molds made from plaster of paris or other gypsum based material leads to a short service life because, inter alia, repeated pressings quickly erodes away the pressing face and tends to break the edges of the mold cavity, causing it to be discarded after only a few hundred pressings. Moreover, the low mechanical strength of the material necessitates the use of heavy metal casing to prevent the mold from cracking during a pressing operation; in many instances molds are lost in the first pressing due to this problem.
The obvious disadvantages of porous molds based on gypsum based materials has caused prior art workers to attempt to develop porous molds which have porosity and permeability characteristics similar to molds made out of plaster of paris but which have good mechanical strength, abrasion resistance, thermal shock resistance, and low water solubility. To date, however, even though many patents have been issued on porous molds, none, to our knowledge, have been placed in commercial production. The reason for this is probably because molds having higher mechanical strength than molds made of plaster and gypsum based materials do not have the requisite porosity and permeability and, most importantly, have surface flaws which ruin the mold for pressing purposes.
It should be noted that a mold merely possessing proper pore size distribution is not necessarily adequate as a mold where the ware is released by purging with air because it is absolutely necessary to have inter-connected pores in order to have purging action equivalent to plaster of paris or gypsum materials. Moreover, it is important that during firing of the mold that the fired mold have approximately the same size as its corresponding unfired mold.